In general, valve seat inserts for internal combustion engines are required to have high wear resistance and high heat resistance. For this reason, sintered alloys have widely been used as a material for the valve seat inserts since there is a wide choice of materials and these sintered alloys make it easy to produce valve seat inserts with excellent performances. Most sintered alloys of this kind contain iron as the main component and have a structure where a hard metal such as an Fe-Mo alloy is dispersed in the pearlite matrix. In these sintered alloys, the strength and the heat resistance are given by the matrix metal, while the wear resistance is given by the dispersed hard alloy. If the sintered alloy is required to have higher characteristics the density of, the sintered alloy is increased by copper infiltration or forging before use.
Recently, as the internal combustion engines are improved in performance, the demand for higher wear resistance and heat resistance of the valve seat inserts has increase. It is, however, dificult with the sintered alloys of the prior art to fill such requirements.
High speed steels are considered to be one material which can meet such requirements. Although the high speed steels have excellent wear resistance and heat resistance, there have some problems in that they have difficulty in machining and are high in the material cost since they require use of expensive elements.
In the Patent Gazette of publication No. 58-39222 or laying open NO. 61-52347, a high Cr sintered alloy having high density increased by liquid sintering and containing Cr carbide dispersed in the matrix is described as a cheap wear-resistant alloy as compared with the high speed steels.
The inventors and other joint inventors proposed, in the specifications of patent applications Ser. Nos. 59-121301 (the Patent Gazette of laying open No. 61-561) and 59-121302 (the Patent Gazette of laying open No. 61-505), valve seat inserts with a double layered construction of which two layers have a composition different from one another. Furthermore, we proposed a valve seat insert infiltrated with copper in the specifications of patent applications Ser. Nos. 58-120457 (the Patent Gazette of laying open No. 60-13062) and 58-124058 (the Patent Gazette of laying open No. 60-13055).
The aforesaid high Cr sintered alloys are excellent in the resistance to pitching wear and the resistance to scratching wear, but their functions are insufficient in terms of the resistance to slipping wear. Thus, they are unsuitable for the parts subjected to not only the pitching wear but also the slipping wear, such as valve seat inserts, because of insufficient wear resistance.
From the investigations on the reason why high Cr sintered alloys with sufficient slipping wear resistance cannot be obtained, it has become clear that the high Cr sintered alloy containing C is generally sintered in the region where a liquid phase and a solid phase coexist and forms hard Cr carbide which is expected to contribute to the improvement of the wear resistance, but the Cr carbide produced in the Fe-Cr-C sintered alloy has small particle size of not more than 20 .mu.m, thus making it impossible to obtain sufficiently high wear resistance. As a method for improving the wear resistance, it is therefore considered to promote the grain growth of the Cr carbide to be produced in the matrix by use of higher sintering temperature or longer sintering time. However, it has also become clear that this means sets limits to the grain growth and results in a decrease in strength of the matrix.
On the other hand, the valve seat inserts with a double layered structure involve complex manufacturing steps, thus making it impossible to avoid producing expensive products.